1. Field Of The Invention
This invention relates to an improved expansion assembly for mine roof bolts which are utilized in relatively small diameter bore holes. The expansion assembly provides greater gripping forces on the inside of the bore hole in the mine roof than can be obtained by similar prior expansion assemblies. The expansion assembly of the present invention may be utilized with or without resin bonding materials.
2. Description Of The Prior Art
It is well-known in the art of mine roof control to tension bolts anchored in bore holes drilled into the mine roof in order to reinforce the unsupported rock formation above the roof. Conventionally a hole is drilled through the roof into the rock formation. The end of the bolt in the rock formation is anchored either by engagement of an expansion assembly on the end of the bolt with the rock formation, or by bonding the bolt with resin to the rock formation surrounding the bore hole, or by use of both an expansion assembly and resin together to retain the bolt within the hole. Examples of an arrangement utilizing both an expansion assembly and resin to anchor a mine roof bolt are disclosed in U.S. Pat. No. 4,419,805, U.S. Pat. No. 4,413,930, U.S. Pat. No. 4,516,885 and U.S. Pat. No. 4,518,292. Other examples of utilizing both an expansion assembly and resin to anchor a mine roof bolt are shown in U.S. Pat. No. 3,188,815, U.S. Pat. No. 4,162,133, U.S. Pat. No. 4,655,645 and U.S. Pat. No. 4,664,561.
Expansion assemblies for roof bolts have been utilized for many years without resin being utilized in the same installation. There have been countless efforts to improve the configuration of various components of the expansion assemblies to provide better anchoring within the bore hole. When mechanical anchor assemblies are utilized in conjunction with resin bonding material, additional modifications are often made to accommodate the resin bonding material. U.S. Pat. No. 4,764,055 discloses an expansion assembly which has been modified in many respects to accommodate the use of resin bonding material with the mechanical expansion assembly.
The use of a mechanical expansion assembly and resin bonding together in the same bore hole produces a roof bolt whose anchorage depends upon both the characteristics of the expansion assembly and the characteristics of the resin. The use of the resin tends to mask the characteristics of the expansion assembly and many inferior expansion assemblies have been utilized with resin since the resin bonding enhances the mechanical expansion assembly to the extent that inferiorities in the expansion assembly are not readily discernible.
For many applications, it has been found to be desirable to use small diameter bore holes within which to install mine roof bolts. When small diameter bore holes are utilized, the power required to drill the hole is greatly reduced, the size and weight of the drill steel is reduced and the size and weight of the roof bolt utilized in the small diameter hole is also reduced, providing for cost savings and more efficient roof control. Bore holes having a nominal diameter of one inch are considered to be small diameter bore holes and such bore holes are often utilized with resin bonding where no mechanical expansion assembly is provided on the roof bolt.
We have determined that a roof bolt with a mechanical expansion assembly can be very effectively utilized in a one inch bore hole if the mechanical expansion assembly is properly arranged to provide strong gripping action on the inside of the bore hole.
There are, on the market, many types of expansion assemblies that are manufactured with a tapered plug and expansion leaves so that as the tapered plug is threaded into the mine roof bolt, it urges the expansion leaves radially outwardly to grip the interior of the bore hole in which the mine roof bolt is inserted. These earlier expansion assemblies are of two general types. One type has a ring to which are affixed several upwardly extending expansion leaves. The ring surrounds the bolt and the tapered plug moves downwardly toward the ring as the assembly is expanded. Another general type of expansion shell is a bail-type shell in which two expansion leaves are supported by a bail that extends over the end of the mine roof bolt and prevents the expansion leaves from moving axially relative to the bolt. The present invention is directed to an improved expansion assembly that has several upwardly extending expansion leaves affixed to a ring to form an expansion shell. U.S. Pat. No. 4,764,055 discloses this general type of expansion assembly. We have taken a commercially available expansion assembly that has been utilized in bore holes and modified its components so that greatly improved gripping power is generated by the expansion assembly when it is utilized in the bore hole of a mine roof. At the same time, this improved expansion assembly has also been modified so that it may optionally be utilized with resin bonding. It need not, however, be utilized with resin bonding and provides greatly enhanced holding power even if no resin bonding is utilized in conjunction with it.